To just make it playable you can just have any bunch of people with suitable tools make placeholder art, a sprite that says "TANK" on it is good enough for the tank moving code to run.
But that looks really bad, and to make it look good (as almost any even mid-budget video game looks) you need an art director who can deliver consistency. For a small game that art director might also just make all the art, so it ends up consistent that way, but games even from the C&C era are too big for that to be realistic, so now you've got several (and maybe hundreds of) volunteers making art and they need to obey that director's stylistic vision or the result is something that lay people can see is "wrong" but they aren't sure exactly why.
There's a little element of this in the programming side. One guy who stubbornly insists on calling loop variables 'x' another insists they should be named 'k' and three others who don't care can already get heated. Or maybe you've run into somebody who believes code should use character U+0009 for indentation instead of using U+0020 as god intended. But the players won't notice that part, whereas if the art isn't consistent that's very visible and detracts from the experience.
"These shall ye use of all that are in the charcodes: whatsoever hath US-ASCII in the keyboards, in the memories, and in the disks, them shall ye use.
And all that have unicodes in the keyboards, and in the memories, of all that move in the disks, and of any living thing which is in the extended ranges, they shall be an abomination unto you."
That effect is pretty visible even in the successful open projects like Wesnoth. Love it to death, like much of the art in isolation, but sometimes you get pretty jarring clashes in style.
But that looks really bad, and to make it look good (as almost any even mid-budget video game looks) you need an art director who can deliver consistency. For a small game that art director might also just make all the art, so it ends up consistent that way, but games even from the C&C era are too big for that to be realistic, so now you've got several (and maybe hundreds of) volunteers making art and they need to obey that director's stylistic vision or the result is something that lay people can see is "wrong" but they aren't sure exactly why.
There's a little element of this in the programming side. One guy who stubbornly insists on calling loop variables 'x' another insists they should be named 'k' and three others who don't care can already get heated. Or maybe you've run into somebody who believes code should use character U+0009 for indentation instead of using U+0020 as god intended. But the players won't notice that part, whereas if the art isn't consistent that's very visible and detracts from the experience.